The present invention relates in general to sensors, such as vortex shedding flowmeters, which utilize fiber optic readouts, and in particular to a new and useful method and apparatus of utilizing the fiber optic readout for operations in a two-wire, 4 to 20 mA format.
A microbend fiber optic sensor unit can be used in a vortex shedding flowmeter. In such flowmeters, an optical cable is held between microbend jaws. One of the jaws is connected to a sensor beam which is exposed to a flow of fluid that has fluid vortices generated therein. The frequency of the fluid vortex generation (called shedding) is a measure of the flow rate for the fluid. Each time a vortex is shed, the sensor beam is moved. This movement is transferred to the microbend jaws which then bend the optical cable or fiber. In this way light which is passing through the optical cable is modulated thus giving a signal corresponding to the passage of the vortex.
Vortex shedding flowmeters using a light barrier which comprises a light source and a spaced apart light detector, is known for example from U.S. Pat. No. 4,519,259 to Pitt et al. U.S. Pat. No. 4,270,391 to Herzl discloses an electronic arrangement for processing signals from a vortex shedding flowmeter.
For any sensor, voltage and/or current signals from the sensor must either be compatible with circuitry for interpreting the signal, or be converted into signals which are compatible.
One industrially accepted transmission path for conveying signals from a sensor or transducer to interpreting circuitry is a two-wire analog transmission system.
Two-wire analog transmission systems are well known. Such systems include a transmitter which is connected to a power supply by two wires which form a current loop. The transmitter includes, as at least one of its features, a transducer or sensor which senses a process variable such as flow rate, pressure or temperature.
The power supply is connected to the two wires to close the current loop. It is also conventional to provide a resistor in the current loop. The transmitter amplifies the signal from its transducer and this amplified signal is used to draw a certain current from the power supply which is proportional or otherwise related to the process variable. It is conventional to draw from a minimum of 4 mA to a maximum of 20 mA. The current between 4 and 20 mA passes through the resistor to produce a voltage drop across the resistor. This voltage drop can be measured to give a value for the process variable.
The electronics for a two-wire, 4-20 mA industrial control transmitter, however, has only about 3.5 mA and 10 volts with which to operate. Fiber optic systems presently require several mA for the light emitter, often 200 mA or greater and as such are not compatible with two-wire, 4-20 mA transmitters.
Although the current drawn by the transmitter goes up above the 4 mA minimum as the process variable being measured changes, present transmitters only use the 4 mA to operate their circuitry and sensor. An additional 16 mA is available at the upper end of the signal range if the circuitry is capable of utilizing it.